Oxidation and sinter-resistant metal powders and pastes

ABSTRACT

A high density precious metal powder that resists sintering at 600° C. and contains a very small percentage of titania or zirconia is produced by co-nucleation and precipitation of gold and titania from an ammoniacal solution at pH 3-4 with a bisulfite. 
     A nickel powder including a very small percentage of a refractory oxide such as zirconia is prepared by co-nucleation and precipitation from an alkaline solution with hydrazine. 
     Materials so prepared have oxidation resistance and sintering temperatures superior to the pure metal, and find utility in microelectronics as air-fired thick films. Nickel powders so prepared are also useful as hydrogenation catalysts and in other applications.

RELATED APPLICATIONS

This is division of application Ser. No. 565,835, filed Apr. 7, 1975,now U.S. Pat. No. 3,966,463, which is in turn a continuation-in-part ofU.S. application Ser. No. 496,714 filed Aug. 12, 1974, and nowabandoned.

BACKGROUND OF THE INVENTION

In the above-noted co-pending application, there is disclosed a methodof co-nucleating precious metal powders with a small percentage of arefractory oxide, most particularly titania or zirconia. The processinvolves a slow precipitation from an acidic solution with ammoniacalhydrazine, followed by a heat treatment. Such powders exhibit increasedsintering temperatures, and find utility when formulated as pastes, witha small amount of glass frit as binder, in thick film electronicapplications, and in catalysis.

The present invention is based, in one part, on the discovery that witha somewhat altered procedure, the process described above can be appliedto nickel and the product exhibits a significantly increased oxidationresistance. The present invention is based, in a second part, on thediscovery that with an essentially different process, precious metalpowders of both increased density and higher sintering temperatures areproduced.

In the above-mentioned co-pending application, it is noted that theaddition of refractory oxides as distinct ingredients in certain thickfilm compositions is known, that the addition of copper oxide to gold tomake a so-called "fritless" thick film conductors is known, and that itis also known to improve bulk metal properties such as creep strengthand tensile strength with a refractory oxide as a distinct second phase,e.g. dispersion hardening. The surprising and novel aspect of thepresent invention, as well as that described in said co-pendingapplication, is the significantly improved thermal properties of metalpowders produced in accordance herewith.

OBJECTS OF THE INVENTION

A general object of the present invention is to provide improved metalpowder compositions.

Another object of the present invention is to provide improved thickfilm pastes.

A further object of the present invention is to provide improved metalfilms for electronic and catalytic applications.

A still further object of the present invention is to provide metalpowder compositions including small amount of co-nucleated ceramicoxides.

A more specific object of the present invention is to provide a preciousmetal powder of both higher sintering temperature and higher densitythan powders heretofore available.

Another specific object of the present invention is to provide nickelpowders including small amounts of a co-nucleated oxide, mostparticularly zirconia, which exhibits improved oxidation resistance.

Yet another object of the present invention is to provide an improvednickel catalyst material.

Various other objects and advantages of the invention will become clearfrom the following description of embodiments thereof, and the novelfeatures will be particularly pointed out in connection with theappended claims.

DESCRIPTION OF EMBODIMENTS

A high density precious metal powder is prepared by co-nucleation andprecipitation of the metal and titania or zirconia from an ammoniacalsolution maintained at a pH in the range of 3-4 with sodium bisulfite.As starting materials, chloroplatinic acid, palladium chloride,chlorauric acid or similar solutions are used. For the refractory oxidecomponent, titanium tetrachloride or disulfato zirconic acid arepreferred.

In the above-noted co-pending application the precipitation is carriedout with hydrazine hydrate and ammonium hydroxide, and the resultingproduct must be fired at around 650° C. to achieve desired results. Bycontrast, the present invention precipitates the metal-oxide combinatiowith a bisulfite, and there is no requirement for firing to achieve bothhigh density and increased sintering temperature. The specific procedurefor preparing a high density gold powder in accordance with theinvention is set forth below in Example I.

The procedures for preparing nickel powders in accordance with theinvention are similar in general to the procedures disclosed in thenoted co-pending application for precious metals, but with some specificchanges. More particularly, while the nickel powder must be fired, thisis done in hydrogen or another reducing atmosphere to prevent nickeloxidation. Because of this firing, it is preferred to use zirconiarather than titania as the oxide component, because the former hasgreater resistance to reduction in the hydrogen environment. As in theprior co-pending application, the reaction rates must be controlled, andare very important to achieve the desired results, but in this case theymust be quite significantly slower.

Generally, a nickel powder in accordance with the present invention isprepared as follows.

A first solution is prepared containing the metals in desiredproportions. This can conveniently be nickel chloride and disulfatozirconic acid. A second solution is then prepared containing sodiumhydroxide, hydrazine hydrate and water. The latter is heated to about90°-95° C. under conditions of agitation. Because the precipitate willbe magnetic, a propeller rather than a magnetic stirrer should be used.The nickel-zirconia solution is then added thereto very slowly,essentially drop-by-drop, at a rate of no more than about 10 ml/min.Agitation is continued until all gas generation has ceased. Theprecipitate is filtered out, dried, pulverized, washed to removeresidual chloride, dried again, screened and fired in hydrogen for abouta hour at 400°-500° C. A specific example of nickel powder production isset forth below in Example II.

To appreciate the improved properties of powder produced in accordancewith the invention, it is necessary to be aware of certain conventions.Thus, when one speaks of powder density, the reference is not to bulkdensity, but rather the tap density, a standard measure for suchpowders. A conventional gold powder will have a tap density of about 4gm/cc; a gold powder produced in accordance with the prior co-pendingapplication had a tap density of 5.2 gm/cc; the powder product ofExample I below had a tap density of 6.7 gm/cc. It is to be noted that ahigher tap density improves thixotropic and rheological properties ofthick film pastes.

As used herein, the word sinter or sintering temperature also has aspecical meaning. After powders are formulated into thick film pastes,usually with a glass frit as binder, they are screened onto ceramicsubstrates in a desired pattern, dried and fired. The film is generallyof the order of one mil thick. A conventional firing cycle for aplatinum-palladium conductor paste is 10-20 minutes at 800°-850° C. inair. If the firing temperature is too high for a particular composition,however, the film will sinter, by which is meant that it beads, ballsup, crazes or, in essence, is other than a smooth, continuous metallicfilm with a good finish. While the sintering temperature is a functionof the melting point of the metal or alloy, it can be very substantiallylower; pure gold powders that sinter at 400° C. are known, whereas agold in accordance with the present invention showns no sign ofsintering when fired at 600° C. This is important in thick film workbecause it affects the choice of glass used in the paste, which mustmelt at the firing temperature, and also dictates the maximumtemperature at which other processing steps can be carried out.

As noted hereinabove, nickel powders according to the present inventionexhibit increased oxidation resistance, but this term too must beconsidered in the proper context. Oxidation properties of a metal arenormally those of the bulk metal, but it must be appreciated thatoxidation is also a function of particle size, and resistance tooxidation drops with particle size due to surface energy factors, (i.e.magnesium metal is stable but in powder form it is explosivelypyrophoric). Thus, standard texts will advise that nickel oxidizes toNiO in air between 350° and 700° C. at a parabolic rate. However, withnickel powders of the size here under consideration, those having aboveabout 30 m.sup. 2 /gm surface area, one would expect rapid andsubstantial oxidation in air at room temperature. The fact thatco-nucleated nickel powder in accordance with the present invention canwithstand 1 hour in air at 450° C. with no measurable weight gain isthus truly remarkable.

Understanding of the invention will be facilitated by referring to thefollowing specific examples thereof, which are illustrative only and arenot to be interpreted in a limiting sense.

EXAMPLE I

It was desired to produce a gold powder including 0.5% by weight oftitania. A solution of the following composition was prepared:

    ______________________________________                                        Chlorauric acid (HAuCl.sub.4) (0.3gmAu/ml)                                                                166 ml                                            titanium tetrachloride in 3N HCl (TiCl.sub.4)                                 (0.25 gmTi/ml)              6 ml                                              DI water                    338 ml                                            ______________________________________                                    

The gold solution is poured, over a span of 1 minute, into stirred,diluted-ammonia, containing 110 ml. of 28-30% NH.sub. 3, and 780 ml.DI-water. A clean 2000 ml. pyrex beaker serves as the reaction vessel.After being stirred an additional 5 minutes, the ammoniated gold slurryis neutralized (pH 6-7) with 3N-HCl. For reduction to metallic gold,this slurry is heated to 50° C., preferably with a quartz or pyreximmersion heater, and then 10 gm. of anhydrous sodium acetate is addedand dissolved. With the source of heat removed, 40 gm. of anhydroussodium bisulfite is added at once to the stirred mixture. Thetemperature rises to about 60° C., and the pH drops to and remains at 4throughout the reduction. The product coagulates in about 15 min., butstirring is continued for 15 min. more, while the reaction temperatureis maintained at about 55° C., using the immersion heater if necessary.After being washed several times with DI-water by decanting, the productis further dispersed and densified in a 1 quart commercial WaringBlender. The wet product is added to the blender containing 100 ml.DI-water and 1.25 gm. cyanoguanidine. After blending at low speed twicefor 30 seconds, the contents are poured into a 400 ml. beaker forsettling and decanting. The gold is then dried at 125° C. and screenedthrough a 140 mesh sieve. Yield is about 90%. Tapped packing density is6.7. Average particle size (Fisher Subsieve Sizer) is 4.1. Surface area(BET) is 0.8.

EXAMPLE II

It was desired to produce a nickel powder containing 2% ZrO.sub. 2.

A solution containing 162.5 ml. of 6.25 N--NaOH, 32.5 ml. of 85%hydrazine hydrate, and 55 ml. DI-water is heated at 90°-95° C. in a 1000ml. beaker, which is fitted with a polypropylene propeller mixer. To thehot solution is added from a separatory funnel, at a rate of 10 ml. perminute, a solution of nickel chloride, containing 24.5 gm. Ni, anddisulfatozirconic acid, equivalent to 0.5 gm. ZrO.sub. 2. After theaddition, mixing is continued for about 15 minutes at 90° C. The mixtureis filtered without washing through a 600M glass buchner funnel, thendried on the funnel at 125° C. The mass is then pulverized in a mortarto pass a 100 mesh sieve. In this condition the product is washable dydecanting with hot DI-water until free of chloride. After drying at 125°C. and screening through 100 mesh, the product is placed in a shallowgraphite boat and fired in a hydrogen furnace at 450° and at 600° C. Theresulting powder properties at these firing temperatures are set forthbelow:

    ______________________________________                                                         450°                                                                             600°                                        ______________________________________                                        Loose Packing Density:                                                                          1.5 gm/cc   1.8 gm/cc                                       Tapped Packing Density:                                                                         1.9 gm/cc   2.4 gm/cc                                       Surface Area     42.7 m.sup.2 /gm                                                                          30.4 m.sup.2 /gm                                 Average Particle Size                                                                           1.46 μm  2.38 μm                                      ______________________________________                                    

EXAMPLE III

The product of Example II was heated in air for 1 hour at 450° C. andgained no weight, indicating significant resistance to oxidation.

Various changes in the details, steps, materials and arrangements ofparts, which have been herein described in order to explain the natureof the invention, may be made by those skilled in the art within theprinciple and scope of the invention as defined in the appended claims.

What is claimed is:
 1. The method of preparing finely divided nickelpowder of increased oxidation resistance comprising:forming an acidicsolution of nickel and a minor proportion of a zirconium oxideprecursor; forming a alkaline hydrazine reducing solution; adding saidacidic nickel solution to said reducing solution very slowly underconditions of agitation; separating, washing and drying the resultingprecipitate; firing said precipitate in a reducing atmosphere at about450° C.
 2. The method as claimed in claim 1, wherein said zirconiumoxide precursor is disulfato zirconic acid.
 3. The method as claimed inclaim 1, wherein the amount of said zirconium oxide precursor issufficient to provide no more than about 2% zirconia in said powder 4.The method as claimed in claim 1, wherein said addition of said nickelsolution is carried out at a rate of no more than about 10 ml. perminute.